The invention relates to potassium-containing polyphosphate glass compositions with low levels of water-insoluble compounds and to methods for their preparation. The invention particularly is directed to polyphosphates having a ratio of potassium to sodium that is relatively high and a relatively low amount of water-insoluble impurities and methods for their preparation.
Sodium salts of phosphoric acid derivatives, especially food grade sodium hexametaphosphate (SHMP), can be used to reduce and control various classes of microorganisms, specifically those important to the safety and spoilage of food and beverages during processing, storage, and distribution. Because these compositions control pathogens at near neutral pH conditions using very dilute preparations, the physicochemical changes (flavor, texture, color, nutrient retention) that may occur at highly alkaline or highly acidic conditions are reduced. For example, sodium salts of phosphoric acid derivatives have been used with preservatives to enhance the potency of preservatives so that lower levels of preservatives can be used, thus improving taste (see, e.g., Calderas (U.S. Pat. No. 5,431,940) and Pflaumer (U.S. Pat. No. 5,641,532), which disclose the use of sodium-containing polyphosphates in combination with sorbate preservatives in dilute juice beverages having relatively low water hardness).
Notwithstanding the antimicrobial benefits, the addition of such sodium salts to foods and beverages increases their sodium content and makes them unacceptable to users who require a low sodium intake. Potassium metaphosphate (KMP), [KPO3]n, (which is also know as Potassium Kurrol's salt) is a crystalline polyphosphate that contains potassium instead of sodium as the metal cation. Because potassium polyphosphate salts, including KMP, contain negligible levels of sodium, their use would substantially reduce the amount of added sodium to foods. But such compositions have not been widely utilized in food applications because they are difficult to dissolve in water and contain relatively large amounts of insoluble material. Technically, KMP is water-soluble but it dissolves so slowly that for practical purposes is considered water-insoluble. Furthermore, it is extremely difficult to produce KMP as a glass or as an easily-solubilized material.
For these reasons, sodium-potassium hexametaphosphate (SKMP), which is a water-soluble mixed-cation analog of sodium hexametaphosphate having significant substitution of sodium with potassium, has the potential to be a desirable alternative to both SHMP and KMP in applications involving polyphosphates (e.g., applications where the functionality of a soluble, longer-chain polyphosphate is desired but without introducing high levels of sodium). Henson et al. (U.S. Pat. Nos. 6,509,050; 6,440,482; and 6,610,340) disclose the use of SKMP in food products as an alternative to SHMP. SKMP typically comprises a distribution of polyphosphate chain lengths and may be represented by the formula (K, Na)(n+2)O(PO3)n, where n designates the average chain length of phosphates. The foregoing formula ignores minor components, including glass-former additives, residual water, water-insoluble impurities (e.g., KPO3), possible ring phosphates, and other trace impurities.
Solid SKMP is a water-soluble glass (or amorphous or vitreous material) that is typically made by chilling a melt containing sodium and potassium phosphates having a mole ratio of cation to phosphorus (M/P mole ratio, where M=alkali metals, typically Na and K but it is possible other alkali metals may be included, and P=phosphorus) that is typically between about 0.9 and about 1.6. SKMP glass made by chilling a melt normally contains an amount of water-insoluble impurity that is typically primarily, if not entirely, KMP. Moreover, the propensity for insolubles to form in glassy SKMP increases with increasing substitution of sodium by potassium. For example, Henson et al. (WO 2001000527) discloses that the formation of insolubles increases substantially as the amount of potassium relative to the total amount of cations increases above about 70-80 mole % M-as-K (mole % M-as-K=100*moles of K/moles of M) and may depend on the chill rate. Iler (U.S. Pat. No. 2,557,109) describe preparation of aqueous solutions of mixed polyphosphates by ion exchange. Although aqueous solutions of mixed polyphosphates may be made by ion exchange, it may be preferred or desirable to make soluble potassium polyphosphates in solid form using a melt process. It still is desirable to further minimize the amount of sodium present in glassy SKMP.
Given the state of the field, a need exists for a water-soluble polyphosphate glass with a relatively high mole ratio of K to Na that also contains a relatively low level of insoluble material. Such a polyphosphate glass would be particularly useful as an antimicrobial agent in food and beverage applications. Furthermore, a need exists for a method of producing such a composition, in particular for decreasing the amount of insoluble content when a relatively high degree of potassium for sodium is used. Further, it is desirable that any method for decreasing the amount of insoluble content be compatible with the conditions currently used for preparing polyphosphates. Additionally, it is preferable for any additive to be a minor component of the resulting composition.